Workflow for a short animated film: what to do first, music or video?
By short animated film I assume it is story driven and that there might be dialog.

The process is this:


write the script (with dialogue)
create storyboard, mood, concept art
record dialogue with video reference of that performance
select Temp music based on script and mood
animate based on script, storyboard, dialogue audio, temp music
compose real music and replace temp music, add sfx


In more detail you can read Pixar's Animation Process.



Why is it like this:

I will skip how script and storyboard is important, the question is not about these.

It is way way easier to animate facial and body expressions if you have dialogue and actor reference as source.

The final camera motion, animation and film cuts are also good sources for the music. However it is easier to cut footage to music than to cut sound to video. The reason is music has rhythm, which the footage might initially not correspond. Speeding/slowing footage and cutting it is easier than doing it with sound.

You solve this paradox by using Temp music, which is just some music already created, that you "steal" and use as reference to time your animations and to use as temporal guide for mood.

This temp music makes it a bit easier to get inspired for flow, mood and timing while animating (similar as with the dialogue audio). But it is no deal breaker if no temp tracks are used.

Making the actual music/score is the last step in creating animated short film.



On a side note:

It is by all means also possible to skip the Temp music part, and source all the flow and mood for animating directly from script and storyboard. The music is then inspired by the visuals (mood-board/story-board) and story emotions (animation/script) and is less restricted.

Temp music can be controversial, because your final score might sound like a knock-off. Because of this, it might be good to change the tempo or roughly re-mix the source for using as temp track.



More matter to watch:

Temp music from composer's point of view: video here

Temp music in film: video here